But unlike in those leaks, "water is unlikely to have reached the ocean as there is no drainage in that tank area," Masayuki Ono, a spokesman for the utility, also known as Tepco, told Reuters. The water that did reach the ocean from the earlier leaks was not considered a widespread threat as it becomes diluted by the vastness of the Pacific Ocean.

The new leak happened late on Wednesday, when water overflowed from a storage tank at the plant after a valve had been left open by mistake. As a result, an excessive amount of water went into a separate holding area, Reuters reported.

The plant has been criticized for its reaction to the three nuclear disasters that followed the quake and tsunami at the site, 220 km (130 miles) north of Tokyo.